Girlfriend! You have responded to my posts as well and we are in such similar circumstances. I am so sorry you have to deal with an asshole of an ex also! Much of what you are going through I have also experienced with my ex and I always feel so bad for my kids when things get heated or ugly. I feel bad for them too when their dads don't see how the things they do affect our kids.
When I get him back from a visit with his dad he's full of anger and he has taken to screaming & hitting when he has to do something he doesn't want to do.
The above statements definitely tell me that you are right in your assumption that your son is not comfortable expressing his feelings and true emotions with his dad. He is comfortable doing so with you because he knows you will love him no matter what. That's a good thing. I know transition times are hard. They have gotten easier over the years with my kids. When they come back now it's pretty much life as usual but it used to be high drama! The confrontations are fewer and things have gotten somewhat better. Definitely not perfect though!
It sounds to me like your son is like my son. He needs to be in one home with more stability in his life. My daughter has handled going back and forth quite well over the years but my son (who is now 6) cannot handle it. He is not happy when he is going back and forth. My daughter is with her dad 1/2 time while my son, who requires more routine, a more frequent and constant caregiver and less change in his life is with me most of the time. He was getting sick way too much with two different schedules and his dad was unwilling to work with me on trying to make things more consistent. Now my son only goes to his dad's every other weekend, 1 weeknight a week and 1/2 the vacations. It has made a world of difference. He has done much better since I had this enforced. I think boys need the consistency more and they also need their moms more when they are younger.
I would have freaked too if my ex took my child to have his or her back adjusted! You do have joint legal custody right? If so, he has definitely broken the rules there... I am not even allowed to take my daughter to a therapist without my ex's signature because we have joint legal custody. Medical guidelines are not spelled out in our decree but it is the law in California. A reputable therapist will not see a child without the signature of both parents if you have joint legal custody. I don't see why a accupuncturist would be any different. I would give that accupuncturist a call and ask why he/she treated your child without legal consent.
Personally, I have to deal with two ex's. The one where I have our child full-time is much easier. He does better in school, he is healthier and his father and I get along much better (because I am willing to communicate for my son's benefit). The one where we have joint custody is much rockier. Communication is strained, it is a constant power struggle, she is having a harder time in school, etc. If you can afford to get full custody I would go for it! It sounds like your son needs it. If you take him to court they will definitely see that you are considering your child's interests and he is not. By not communicating with you he is hurting your child.
In my situation with my daughter, I choose the less aggressive path with my ex. I pick my battles carefully. I cannot afford to take him to court and I want to keep the lines of communication open. It helps to a point.
In the meantime, breath and keep the communication to a minimum. No sense in fighting over things that are out of your control at the moment. Save it for your attorney. That's what I would do if I were taking my ex to court. Document it all and take some peace in the fact that you are doing what is best for your child and that is all you are capable of.